The present invention generally relates to an optical element and to a method of making an optical element. More specifically, the present invention relates to an optical element that incorporates a functional property and to a method of incorporating the functional property in the optical element.
Manufacturers have encountered and overcome numerous technical challenges presented by chemical compounds that are newly incorporated into optical elements, such as ophthalmic lenses. First, there were new formulations for inorganic glasses. These new glass formulations required development of new processing steps and conditions, as well as, new grinding and polishing techniques. Later, the industry focus shifted to thermoset plastics, such as allyl diglycol carbonate, which is sold under the CR-39.RTM. trademark of PPG Industries, Inc. One of the more recent developments involves manufacture of ophthalmic lenses using thermoplastic materials, such as polycarbonate.
Polycarbonate is an amorphous, thermoplastic material that has mechanical properties which are very desirable for ophthalmic lenses. For example, ophthalmic lenses made of polycarbonate have unusually high impact resistance and strength which make the lenses surprisingly shatter-resistant. Also, the relatively low specific gravity of polycarbonate makes it possible to significantly decrease the weight of polycarbonate lenses, as compared to glass or CR-39.RTM. lenses. Polycarbonate also is highly transparent and has a desirably high refractive index for ophthalmic lens applications. Furthermore, the good thermal properties of molten polycarbonate makes the material conducive to efficient processing by conventional techniques, such as injection molding.
Despite the advantages of the material, incorporation of polycarbonate into ophthalmic lens manufacture has not been without problems. For example, the hardness of the material necessitated development of different grinding and polishing techniques from techniques used for glass and CR-39.RTM. lenses. Additional challenges remain that have not been satisfactorily addressed to date. These challenges relate to incorporation of functional properties into lenses made of polycarbonate. Functional properties include features, such as filtering, cosmetic and durability features, that may be incorporated into an optical lens to impart or modify lens characteristics, other than optical power or magnification characteristics. Some examples of functional properties include light polarization, photochromism, tint, color, decor, indicia, hardness, and abrasion resistance.
There are various examples of shortcomings relating to polycarbonate ophthalmic lenses and to techniques for manufacturing polycarbonate ophthalmic lenses. For example, no method presently exists for making polarized polycarbonate lenses capable of meeting ophthalmic prescription specifications--that is, creating polarized polycarbonate lenses with different focal powers. Also, no method presently exists for quickly and efficiently making high quality photochromic polycarbonate lenses. Furthermore, no method is available for efficiently and effectively incorporating functional properties into ophthalmic lenses made of new materials or materials that have not yet been adapted to ophthalmic lens manufacture.
Numerous methods for incorporating polarizing properties into lenses made of material other than thermoplastic material are known. For example, U.S. Pat. No. 3,051,054 to Crandon describes a method of providing a glass lens with a film of light-polarizing material. Also, U.S. Pat. No. 4,495,015 to Petcen describes a method of laminating a thermoset/thermoplastic wafer to an ophthalmic glass lens.
Various patents disclose methods of incorporating a polarizing film or wafer within a lens cast of thermoset material. For example, U.S. Pat. No. 3,786,119 to Ortlieb discloses a laminated plate of polarizing plastic material that is formed into a polarizing screen. The polarizing screen is placed within a mold which is filled with polymerizable or polycondensible liquid resin. U.S. Pat. No. 3,846,013 discloses a light-polarizing element formed by sandwiching light-polarizing material between thin layers of optical quality transparent polymeric material. The light-polarizing element is placed within a mold, and a polymerizable monomer is placed in the mold on either side of the light-polarizing element.
Also, U.S. Pat. No. 3,940,304 to Schuler discloses a shaped light-polarizing synthetic plastic member that is disposed between layers of an optical quality synthetic monomeric material in a mold. A monomeric material is placed within the mold and polymerized to form a composite synthetic plastic light-polarizing lens structure. U.S. Pat. No. 4,873,029 to Blum discloses a plastic wafer that may include polarizing features. The plastic wafer is inserted into a mold between liquid monomer molding material. The mold is then subjected to oven-curing to polymerize the liquid monomer. Additionally, U.S. Pat. No. 5,286,419 to van Ligten et al. discloses a shaped polarizing film that is embedded in pre-gelled resin. The resin is cured to form a light polarizing lens.
However, despite the availability of these methods for incorporating a polarizing film or wafer within a lens cast of thermoset material, a need remains for an improved polarizing lens. For example, delamination of cast polarizing lenses remains a significant problem. Also, cast lenses are relatively heavy and offer less than adequate levels of impact and shatter resistance. Finally, manufactures continue to encounter difficulties making polarizing cast lenses with optimum refractive index values.
Another reference, U.S. Pat. No. 5,051,309 to Kawaki et al., concerns a polarizing plate that is made by laminating polycarbonate film on both sides of a polarizing thin layer. The polarizing thin layer is composed of a polymeric film and a dichroic dye oriented on the polymeric film. According to the patent, suitable uses of the polarizing plate include goggles and sunglasses. However, the polarizing plate of U.S. Pat. No. 5,051,309 would not be suitable for use as an optical lens capable of meeting ophthalmic prescription specifications. For example, the polycarbonate film included in the polarizing plate of this patent lacks the material integrity needed for successful grinding and polishing of polycarbonate optical elements to prescription specifications. Polycarbonate that is ground and polished to make optical elements must have sufficient material integrity to withstand the heat and pressure generated during grinding and polishing operations. The lack of material integrity of the polycarbonate film used in the Kawaki polarizing plate would affect cosmetic properties, as well as, the impact strength of any prescription specification optical elements made by grinding and polishing the polycarbonate film.
As noted, another challenge concerns incorporation of photochromic properties into polycarbonate lenses. For example, present polycarbonate lenses that include photochromic material offer marginal, and even unacceptable, photochromic properties and cosmetic qualities. Indeed, no method presently exists for making high quality photochromic polycarbonate lenses.
Two current methods of incorporating organic photochromic dyes into thermoplastic materials, such as polycarbonate, involve either inclusion of organic photochromic dye throughout the thermoplastic material or imbibition of photochromic dye into a surface of the thermoplastic material. Existing techniques, such as injection molding, for including organic photochromic dyes throughout thermoplastic materials, such as polycarbonate, typically do not yield satisfactory results. It is believed that the unsatisfactory results occur for several reasons, including the relatively high temperatures required for satisfactory injection molding and including the relatively high glass transition temperatures of many thermoplastics, such as polycarbonate.
For example, naphthopyrans, spironaphthopyrans, and spirooxazines that are co-melted with thermoplastics typically break down when exposed to the relatively high temperatures present during injection molding. This has been found to be especially true when the thermoplastic material is polycarbonate resin. As another example, polycarbonate, has a stiff molecular structure that is reflected by the relatively high glass transition temperature of polycarbonate. Therefore, even in the absence of photochromic compound break down, the stiff molecular structure of polycarbonate would be expected to substantially inhibit full activation of the photochromic dye, since the photochromic dye must go through a geometric transformation in the polycarbonate to activate.
The present inventors conducted an experiment to examine these photochromic compound break down and activation inhibition phenomena. The experiment involved co-melting mixing equal concentrations of an organic photochromic dye into polycarbonate resin and into cellulose acetate butyrate resin. Then, a sheet of the polycarbonate/photochromic dye mixture and a sheet of the cellulose acetate butyrate/photochromic dye mixture were cast. It was observed that the photochromic activity of the photochromic dye in the polycarbonate was approximately one half that of the photochromic activity of the photochromic dye in the cellulose acetate butyrate, under the same conditions of ultraviolet light exposure.
Imbibition of photochromic dyes into surfaces of thermoplastic materials, such as polycarbonate, also yields unsatisfactory results, which are again believed related, at least in part, to the relatively high glass transition temperatures of many thermoplastics, such as polycarbonate. For example, polycarbonate has a stiff molecular structure, as reflected by the relatively high glass transition temperature of polycarbonate. Poor photochromic dye imbibition results obtained with polycarbonate are believed related to the stiff molecular structure of polycarbonate. More specifically, it is thought that the stiff molecular structure substantially prevents the photochromic dye from penetrating into the polycarbonate.
Modification of the surface structure of polycarbonate by treatment with a solvent is said to improve imbibition of photochromic compounds into polycarbonate. In particular, U.S. Pat. No. 5,268,231 to Knapp-Hayes discloses that cyclohexanone is one of the more effective solvents for modifying the polycarbonate surface structure to accept photochromic compounds. However, the present inventors have completed experiments following the methods described in U.S. Pat. No. 5,268,231 and have found that this method leaves the surface of the polycarbonate with a rough, orange-peel type texture that is unacceptable for ophthalmic lenses. For example, the rough texture of the treated polycarbonate causes irregular and unpredictable optical effects in the treated polycarbonate.